Although dwarf carbon ( dC ) stars are thought universally to be binaries to explain the presence of C _ { 2 } in their spectra while still near main sequence luminosity , direct observational evidence for binarity is remarkably scarce .
Here we report the detection of a 2.92 d periodicity in both photometry and radial velocity of SDSS J125017.90+252427.6 , an r = 16.4 dC star .
This is the first photometric binary dC , and only the second dC spectroscopic binary .
The relative phase of the photometric period to the spectroscopic observations suggests that the photometric variations are a reflection effect due to heating from an unseen companion .
The observed radial velocity amplitude of the dC component ( K = 98.8 \pm 10.7 { km } { s } ^ { -1 } ) is consistent with a white dwarf companion , presumably the evolved star that earlier donated the carbon to the dC , although substantial orbital evolution must have occurred .
Large synoptic photometric surveys such as the Palomar Transient Factory , used for this work , may prove useful for identifying binaries among the shorter period dC stars .